Typical hybrid electric vehicles (HEVs) in widespread use have a limited battery capacity; in such systems the vehicle operates on electric-only operation for limited periods of time. The components requiring lubrication are supplied by a mechanical pump coupled to the internal combustion engine. Thus, in electric-only operation, the mechanical pump does not rotate and supplies no oil to components in the oil circuit. It has been found that the amount of oil in the components is sufficient for such limited periods of electric-only operation. In such HEVs, the amount of electric-only operation is limited, though, by how long the components can survive on the residual lubricant in the system.
To further reduce petroleum consumption in HEVs, manufacturers are developing plug-in hybrid electric vehicles (PHEVs). The battery pack on a PHEV has a greater storage capacity and the PHEV is provided with charging capability to charge the battery pack from an electrical grid so that the PHEV derives its power from both the electrical grid and petroleum sources. The duration of electric-only operation in a PHEV is significantly increased in comparison to HEVs with limited battery capacity. The lubrication and cooling needs of power-generating and power-transmitting components in the PHEV are not satisfied by the mechanical pump driven by the internal combustion engine.